The center

In line with my understanding of the Center one appends below the chapters in the Tao Te Ching (TTC) where Laozi had referred to it including how to reach the center through the cultivation of essence (meditation) and bodily life (cardinal virtues).

6 The Center

The valley spirit dies not, aye the same;
The female mystery thus do we name.
Its gate, from which at first they issued forth,
Is called the root from which grew heaven and earth.
Long and unbroken does its power remain,
Used gently, and without the touch of pain.

14 The Meditation

We look at it, and we do not see it, and we name it 'the Equable.' We listen to it, and we do not hear it, and we name it 'the Inaudible.' We try to grasp it, and do not get hold of it, and we name it 'the Subtle.' With these three qualities, it cannot be made the subject of description; and hence we blend them together and obtain The One. Its upper part is not bright, and its lower part is not obscure. Ceaseless in its action, it yet cannot be named, and then it again returns and becomes nothing. This is called the Form of the Formless, and the Semblance of the Invisible; this is called the Fleeting and Indeterminable. We meet it and do not see its Front; we follow it, and do not see its Back. When we can lay hold of the Tao of old to direct the things of the present day, and are able to know it as it was of old in the beginning, this is called (unwinding) the clue of Tao.

18 The Cardinal Virtues

When the Great Tao (Way or Method) ceased to be observed, benevolence and righteousness came into vogue. (Then) appeared wisdom and shrewdness, and there ensued great hypocrisy. When harmony no longer prevailed throughout the six kinships, filial sons found their manifestation; when the states and clans fell into disorder, loyal ministers appeared. (TTC by Laozi [Legge] www.sacred-texts.com)

Notes:
1) The valley spirit and the mysterious female represent the light and the dark. And the gate represents the Tai Chi (supreme ultimate) where Heaven and Earth grew from.
2) Inner seeing and inner hearing are required to see the sights within, which cannot be grasped. Laozi proceeded to describe some of the sights seen by those who constantly practise the breath control meditation known as the "Circulation of the Light" revealed by Guan Yinshi (for whom Laozi is said to have written the TTC) .
3) After the fall from Tao, the ancients also taught the cultivation of virtues for people to return to the Tao. For various reasons, this part of Tao cultivation does not sit well with Western Daoists. Consequently different interpretations and understandings arise.

Sometimes the truth is front of you, inside you or out there.